Add HST/tax support and payment reporting with HST aggregation

Studio Settings gains a default HST rate; the rate is frozen onto each
payment at booking and computed against the pre-tax subtotal, with the
total billed as subtotal + tax. The rate is overridable per booking on
My Lessons while unpaid (recomputing the tax amount), comped
registrations are never taxed, and receipts break out subtotal/HST/total.

Builds the payments report (roadmap #8) from us_payments: a monthly
per-instructor view with subtotal, HST collected, and grand-total
aggregation, plus a nonce-protected CSV export via admin-post. Studio
admins see all instructors and can filter; instructors are scoped to
their own rows. The Payment Report menu is gated on export_payments so
instructors (who lack manage_billing) can reach it.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/Schema.php

@@ -151,6 +151,8 @@ class Schema {
                 currency                 VARCHAR(3)      NOT NULL DEFAULT 'CAD',
                 method                   VARCHAR(20)     NOT NULL DEFAULT 'etransfer',
                 status                   VARCHAR(20)     NOT NULL DEFAULT 'pending',
+                tax_rate                 DECIMAL(5,2)    NOT NULL DEFAULT 0,
+                tax_amount               DECIMAL(10,2)   NOT NULL DEFAULT 0,
                 etransfer_email          VARCHAR(191)    DEFAULT NULL,
                 stripe_payment_intent_id VARCHAR(255)    DEFAULT NULL,
                 receipt_number           VARCHAR(50)     DEFAULT NULL,